1. Field of the Invention
This invention relates to a DC motor control system which provides a constant DC motor output characteristic over a wide range and ensures excellent response of the DC motor.
2. Description of the Prior Art
Where a DC motor having field windings is employed for driving a spindle of a machine tool, the DC motor is usually driven in the range in which its output characteristic is constant.
At low speeds, the DC motor has a constant torque characteristic and produces an output proportional to the number of revolutions. At high speeds, it exhibits a constant output characteristic regardless of the number of revolutions. The output characteristic of a DC motor having a rated output of 15 KW and a revolving speed of 3500 r.p.m., for example, is constant in the range from 1160 to 3500 r.p.m. and has a constant torque characteristic below 1160 r.p.m., as shown in FIG. 1. Accordingly, when this DC motor is used for the above-mentioned spindle drive, it is driven in the range of 1160 to 3500 r.p.m.
However, there is a demand for spindle drives covering the lower speed range and, to meet this demand, a speed change mechanism utilizing a gear, a clutch, etc. is coupled to the DC motor to reduce its revolving speed, for example, to 1/3, thus enlarging the constant output range for the spindle drive. That is, as indicated by the broken line in FIG. 1, the constant output range can be enlarged by the speed change mechanism to cover 380 to 3500 r.p.m. However, the provision of such a speed change mechanism introduces complexity in construction, requires extra space and increases inertia of the load on the DC motor to degrade its response.
Another means of enlarging the constant output range is to drive the DC motor in a manner to produce an output which is 1/2 or 1/3 of its rated output, as depicted in FIG. 2. For instance, if a DC motor having a rated output of 15 KW is driven to provide an output 5 KW, the constant output characteristic can be obtained in the range of from 380 to 3500 r.p.m. However, this method also has the defect of lowered response of the DC motor due to the limitation of its output.